This invention relates to electronic test and measurement instruments and, more particularly, to electronic instrumentation for signal measurements during tests performed on radio frequency (RF) systems, subsystems, and associated components. Specifically, one embodiment of the invention is directed to a method and apparatus for measuring harmonics produced by RF devices, such as amplifiers, being tested, or for measuring conversion loss of mixers being tested, in response to swept frequency stimulii applied by either an internal or external signal source.
Active RF devices require both linear and non-linear characterization. When measuring linear and non-linear characteristics of a device under test (DUT), such as an amplifier or mixer, multiple test configurations are typically needed. This can be time-consuming, because each DUT must be moved between one test set-up to measure such characteristics as impedance, amplifier gain, and mixer conversion loss and another test set-up to measure harmonics.
Considered in more detail, the behavior of a DUT is linear when a sine wave input produces a sine wave output at the same frequency with only an amplitude and phase change. Examples of linear devices are filters and cables. Vector network analyzers have traditionally been used to measure the reflection and transmission characteristics of linear RF devices by applying a known swept frequency signal and then measuring the magnitude and phase of the transmitted and reflected signals. This requires a vector network analyzer configured in a first test set-up.
The output of a non-linear DUT is dependent on the power level of the input signal and is usually composed of multiple signal components at harmonically related frequencies. Examples of such non-linear RF devices are saturated amplifiers and mixers. The majority of measurements are linear, but many applications also require non-linear information. Today, the typical test set-up uses an external RF source to stimulate a DUT at a continuous wave (CW) frequency with a spectrum analyzer connected at the output of the DUT. The output spectrum of the DUT is then displayed. This technique provides high dynamic range and the ability to measure total harmonic distortion (including all harmonics, spurious and intermodulation products). However, in contrast to the use of a vector network analyzer for linear measurements, non-linear measurements are performed with a spectrum analyzer configured in a different test set-up.